Speech coding systems yielding a high quality of coded speech at low bit rates are of increased interest of late. For this purpose linear prediction coding (LPC) techniques are usually used, these techniques exploiting spectral speech characteristics and allow coding only of the preceptually important information. Many coding systems based on LPC techniques perform a classification of the speech signal segment under processing for distinguishing whether it is an active or an inactive speech segment and, in the first case, whether it corresponds to a voiced or unvoiced sound. This allows coding strategies to be adapted to the specific segment characteristics. A variable coding strategy, where transmitted information changes from segment to segment, is particularly suitable for variable rate transmission, or, in case of fixed rate transmissions, allows exploiting possible reductions in the quantity of information to be transmitted for improving protection against channel errors.
An example of variable rate coding system in which a recognition of activity and silence periods is carried out and, during the activity periods, the segments corresponding to voiced or unvoiced signals are distinguished and coded in different ways, is described in the paper "Variable Rate Speech Coding with online segmentation and fast algebraic codes" by R. Di Francesco et alii, conference ICASSP `90, 3-6 April 1990, Albuquerque (USA), paper S2b.5.